The present disclosure relates generally to processes for forming bisphenol-A, such as processes for purifying waste streams resulting from the synthesis of 2,2-bis(4-hydroxyphenyl)propane, also known as bisphenol-A, BPA, para,para-BPA, or p,p-BPA. For example, one embodiment of the disclosure relates to methods for producing bisphenol-A from an impurity, such as chroman 1.5. Also included are various isolated, converted, or purified products formed by these processes.
Bisphenol-A is used as an intermediate in the production of certain polycarbonates, epoxies, phenoxies, polysulfones, and polyester resins. Bisphenol-A itself is commercially produced by the condensation of phenol with acetone in the presence of an acid catalyst and in the presence of a cocatalyst (e.g., a mercaptan) which acts as a reaction rate accelerator. A common method involves the use of a sulfonated cation exchange resin in the free acid form, through which the phenol and acetone mixture is run. The resulting product stream can then be purified to obtain bisphenol-A and a mother liquor containing various byproducts and impurities. The mother liquor is distilled to reduce water. Fresh phenol, acetone, and co-catalyst are then added to the mother liquor and fed to the sulfonated cation exchange resin again.
One of these impurities in the mother liquor is chroman 1.5, also known as 4-(2,4,4-trimethyl-chroman-2-yl)-phenol or 2,4,4-trimethyl-2-(4-hydroxyphenyl)chroman. Depending on conditions, chroman 1.5 can be produced at 0.8 to 1.5 weight percent concentrations. Chroman 1.5 has the chemical structure shown below:

The mother liquor may be further processed to increase the overall yield of the reaction. For example, the mother liquor may be sent to a tar cracker unit to crack the impurities into phenol and other light or heavy molecules. The phenol can then be separated from the heavy molecules (via distillation) and recycled into the bisphenol-A production line. The light molecules remain with the phenol and are also recycled into the bisphenol-A production line.
However, chroman 1.5 co-distills with phenol and is thus difficult to remove via distillation. Although present in low quantities of from 0.1 to 6 weight percent in the phenol, the presence of chroman 1.5 has been found to reduce the lifetime and the selectivity of the acid catalyst used in the phenol-acetone condensation process. In addition, chroman 1.5 has color-forming tendencies at elevated temperatures and its presence is unsuitable in the polycarbonates made from the bisphenol-A, which may need to be transparent and clear.
It would be desirable to remove or convert impurities, such as chroman 1.5, to improve the performance of the acid catalyst system, increase catalyst lifetime, improve the quality of the resulting bisphenol-A, and/or increase the yield by improving the usage of the raw material (i.e. minimizing the amount of waste that needs to be disposed).